1. Field of the Invention
The present invention relates to an ammoxidation catalyst for use in producing acrylonitrile or methacrylonitrile from propane or isobutane by ammoxidation in the gaseous phase. More particularly, the present invention is concerned with an ammoxidation catalyst comprising a compound oxide which contains specific component elements in specific atomic ratios and which exhibits an X-ray diffraction pattern wherein the intensities of two peaks respectively appearing at diffraction angles (2.theta.) of 27.3.+-.0.3.degree. and 28.2.+-.0.3.degree. have a specific relationship to each other. By the use of the ammoxidation catalyst of the present invention, not only can acrylonitrile or methacrylonitrile be produced in high yield, but also oxidative decomposition of ammonia feedstock into nitrogen can be effectively suppressed, thereby enabling an improved utilization of ammonia as a feedstock. The present invention is also concerned with a process for producing acrylonitrile or methacrylonitrile by using such an excellent ammoxidation catalyst.
2. Prior Art
There has been a well-known process for producing acrylonitrile or methacrylonitrile by ammoxidation of propylene or isobutylene. Recently, as a substitute for such a process using propylene or isobutylene, attention has been attracted to a process for producing acrylonitrile or methacrylonitrile by gaseous phase catalytic ammoxidation of propane or isobutane, i.e., by gaseous phase catalytic reaction of propane or isobutane with ammonia and molecular oxygen. Further, a number of proposals have been made with respect to catalysts for use in the ammoxidation of propane or isobutane.
For example, as a catalyst for use in producing acrylonitrile or methacrylonitrile by ammoxidation of propane or isobutane, oxide catalysts containing molybdenum, vanadium, niobium and tellurium are known. Such oxide catalysts are disclosed in U.S. Pat. No. 5,049,692, U.S. Pat. No. 5,231,214, U.S. Pat. No. 5,281,745, U.S. Pat. No. 5,422,328, Unexamined Japanese Patent Application Laid-Open Specification No. 6-227819, Unexamined Japanese Patent Application Laid-Open Specification No. 7-144132, Unexamined Japanese Patent Application Laid-Open Specification No. 7-232071, Unexamined Japanese Patent Application Laid-Open Specification No. 8-57319 and Unexamined Japanese Patent Application Laid-Open Specification No. 8-141401.
Further, oxide catalysts containing molybdenum, vanadium, antimony and niobium are disclosed in European Patent Application Publication No. 767 164 A1 and Unexamined Japanese Patent Application Laid-Open Specification No. 5-213848.
In addition, oxide catalysts containing vanadium and antimony are disclosed in U.S. Pat. No. 4,760,159 and U.S. Pat. No. 4,797,381.
Among the above-mentioned prior art documents, each of U.S. Pat. No. 5,281,745 and U.S. Pat. No. 5,422,328 discloses a crystalline metal oxide catalyst exhibiting an X-ray diffraction pattern having peaks at diffraction angles (2.theta.) of 22.1.+-.0.3.degree., 28.2.+-.0.3.degree., 36.2.+-.0.3.degree., 45.2.+-.0.3.degree. and 50.0.+-.0.3.degree., respectively; Unexamined Japanese Patent Application Laid-Open Specification No. 6-227819 discloses a crystalline metal oxide catalyst exhibiting an X-ray diffraction pattern having peaks at diffraction angles (2.theta.) of 22.1.+-.0.5.degree., 28.2.+-.0.5.degree., 36.2.+-.0.5.degree., 45.2.+-.0.5.degree. and 50.0.+-.0.5.degree., respectively; and Unexamined Japanese Patent Application Laid-Open Specification No. 7-232071 discloses a crystalline metal oxide catalyst exhibiting an X-ray diffraction pattern having peaks at diffraction angles (2.theta.) of 9.0.+-.0.3.degree., 22.1.+-.0.3.degree., 27.3.+-.0.3.degree., 29.2.+-.0.3.degree. and 35.4.+-.0.3.degree., respectively. However, in any of these prior art documents, there is no description about the intensity ratio of peaks appearing at diffraction angles (2.theta.) of 27.3.+-.0.3.degree. and 28.2.+-.0.3.degree., respectively.
The oxide catalysts described in these prior art documents are disadvantageous in that none of them exhibit a satisfactorily high yield of acrylonitrile or methacrylonitrile in the ammoxidation of propane or isobutane.
Applied Catalysis A General (vol. 157, pp.143-172, 1997) describes that, during the course of the ammoxidation of propane, ammonia, which is one of the gaseous feedstocks for the ammoxidation is converted not only to acrylonitrile as a desired product, but also inevitably to by-products, such as acetonitrile and hydrocyanic acid, and, is oxidatively decomposed into nitrogen. The conventional catalysts for use in the ammoxidation of propane or isobutane pose a problem in that, during the ammoxidation, decomposition of ammonia into nitrogen occurs to a high extent. Therefore, it is desired to develop an improved catalyst having an advantage in that the decomposition of ammonia during the ammoxidation can be suppressed, thereby enabling an efficient utilization of the feedstock ammonia for the ammoxidation of propane or isobutane.
On the other hand, U.S. Pat. No. 5,534,650 discloses a method for performing an ammoxidation of an alkane in which gaseous ammonia is fed into a reactor from a plurality of ammonia inlets provided therein so that the fed ammonia is efficiently utilized for the ammoxidation reaction. However, this method is disadvantageous not only in that extensive operations are required due to the use of complicated equipment, but also in that a satisfactory effect for suppressing the ammonia decomposition cannot be obtained.